ContextBar: Move m_freehandBox to the last position to improve context bar for selection tools
Change selection behavior: left-click replace the selection

With this patch we replace the "unique/odd" behavior of Aseprite where
you add selection regions with left-click and remove with right-click.
Now by default you replace the selection with left-click (as in regular
gfx programs). Also you can change the selection tool behavior with
buttons/icons at the context bar (to select between replace/add/subtract).